When hydrogen is used as a fuel in motor vehicles, a hydrogen fuel depot infrastructure for refueling must also be developed. Typically, in the use of hydrogen to fuel motor vehicles, or in the use of compressed natural gas (CNG) to power engines in motor vehicles, present practice is that hydrogen is stored in refillable on board fuel tanks having a maximum design pressure in the range of about 5000 psi, CNG is stored in tanks having a maximum design pressure in the range of about 3600 psi. Pressures exceeding 3600 psi for CNG and 5000 psi for hydrogen and in the range of about 10,000 psi or more are likely to be utilized as the use of high pressure fuel gas technology becomes more widespread in motor vehicle applications. High pressure gas powered vehicles typically utilize light weight reinforced polymer/composite storage tanks to store gaseous fuel on board at high pressure. Herein, reference to hydrogen powered vehicles correlates with the use of the invention with compressed natural gas powered vehicles (CNGVs). When hydrogen is referred to in the specification, that term is intended to be interchangeable, evident in context, with compressed natural gas, high pressure fuel gas, or gaseous fuels in general.
I have previously filed numerous applications for patent in the United States Patent Office relating to various designs for high pressure hydrogen refueling systems—particularly in situ and on board cooling apparatus. Without more, during a high pressure refueling process, the interior of the on board tanks, namely, the gas itself, becomes heated as a result of gas compression as the tank pressure increases and other refueling parameters affect refueling. Conventionally, it is not usually possible to obtain a full refill tank pressure to a high pressure design maximum, for example, 10,000 psi (70 MPa), without a pressure or temperature compensation system during the course of refueling. Namely, the charge of gas fuel input into and stored in the tank must be initially in excess of the optimum design tank pressure because of the gas compression/heating effect caused by the high pressure compression of gas in the tank as a result of refueling. Without some form of compensation or treatment, vehicle mileage in terms of vehicle range is reduced as a result of the compression/heating effect. As higher tank design pressures are utilized, a full tank refill to optimum capacity becomes more difficult and an underfill results.
Solutions have been proposed to resolve this under fill problem encountered with high pressure tanks. A slower flow rate during refill results in a lower initial tank temperature, however, a slow fill, is undesirable, and may be impractical when significant numbers of refuel customers are involved. An undesirable consequence of a slower flow rate during refueling to avoid heat build up is a longer refueling time. Another option is to cool the gas before refueling. Pre-cooling to a very low temperature to ameliorate compression heating effects, however, requires substantial energy, thereby reducing the overall energy efficiency of a high pressure gas infrastructure. A low temperature pre cool may occur in real time simultaneously with the introduction of high pressure gas into the vehicle tank; or a quantity of high pressure gas at the tank farm intended for refueling dispensation may be pre cooled in bulk at the station, and then conveyed to the vehicle tank through the refuel meter. A pressure overfill is another option that requires an additional energy expense in gas compression. Higher pressure, however, exacerbates the heat generated in the tank as a result of higher pressure compression. While secondary pre-treatment of refill gas is generally unnecessary when fill pressures are at 5000 psi or lower, when tank pressures exceed 3600 psi (CNG) and 5000 psi (hydrogen), and approach or exceed 10,000 psi, gas volume or quantity compensation becomes an important factor in the refueling process so that a full tank capacity fill may be achieved.